as_c_string.pl

#!/usr/bin/perl -n

chomp;
next if !s/^\x20{0,3}\S+:\s*//;
my ($code, $listing) = split(/\s*\t\s*/, $_, 2);
$code =~ s/(^|\s)+/\\x/g;
$spaces = ' ' x (30 - length($code));
print "\"$code\"$spaces// $listing\n";

runtime_code_modification.c

#include <stdio.h>
#include <string.h>
#include <malloc.h>
#include <assert.h>
#include <sys/mman.h>

/* This code was generated by using the following commands:
 * $ gcc hello.c -c
 * $ objdump -rD hello.o -j .text | perl format_as_c_string.pl
 */

char program[] =
"\x55"                          // push   %ebp
"\x89\xe5"                      // mov    %esp,%ebp
"\x83\xec\x18"                  // sub    $0x18,%esp
"\xc7\x04\x24\x00\x00\x00\x00"  // movl   $0x0,(%esp)
"\xe8\xfc\xff\xff\xff"          // call   e <print1+0xe>
"\xc9"                          // leave  
"\xc3"                          // ret    
;

int main() {
	char *hello = "Hello world";
	assert(sizeof(void*) == 4); // IA-32 only

	/* Allocating memory */
	char *code = mmap(NULL, sizeof(program), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
	memcpy(code, program, sizeof(program));

	/* R_386_PC32 relocation */
	void * puts_addr_rel = (void*) ((void*) puts - (void*) &(code[18]));
	memcpy(&(code[14]), &puts_addr_rel, sizeof(void*));
	/* R_386_32 relocation */
	memcpy(&(code[9]), &hello, sizeof(void*));

	/* Firing the code execution */
	assert(mprotect(code, sizeof(program), PROT_READ | PROT_EXEC) == 0);
	(*(void (*)()) code)();
}